Full reset to original firmware

Hi, i would want to do a full reset of my WRT54GS to recover the original firmware because my current version (HyperWRT 2.1b) has problems :
- I cannot reach web interface any more to manage my router.
- I'm connect to the Internet (MSN Messenger and some sites functions) but some sites like Google don't function whereas when i ping the site, i've a reply...

So, i thought that i had to put the last firmware officials.
But i can't go to the Web Interface to upgrade firmware and also with Tftp (the software of Linksys), i can't do that.

After 2 hours with the Technical Support of Linksys, i didn't find a solution to resolve my problem. The technician explain me that it need an official firmware to have the guarantee.
So, i want delete the HyperWRT and recup the 1st firmware which i had (v3.37.2).

Miracle, the web interface and the page "Firmware upgrade" function again but when i choose a firmware and try to upgrade it, i've after 1 or 2 minutes "Upgrade are failed ".

But now i can acces to the web interface and so to the Shell, telnet...Indeed, the technician has said me that it would be posible to upgrade or to put at zero the firmware since the Shell. If someone know how i can do that.